Stereospecific formation acid esters

Recently a simplified process was developed for incorporating l-methionine directly into soy proteins during the papain-catalyzed hydrolysis (21). The covalent attachment of the amino acid requires a very high concentration of protein and occurs through the formation of an acyl-enzyme intermediate and its subsequent aminolysis by the methionine ester added in the medium. From a practical point of view, the main advantage of enzymatic incorporation of amino acids into food proteins, in comparison with chemical methods, probably lies in the fact that racemic amino acid esters such as D,L-methionine ethyl ester can be used since just the L-form of the racemate is used by the stereospecific proteases. On the other hand, papain-catalyzed polymerization of L-methio-nine, which may occur at low protein concentration (39), will result in a loss of methionine because of the formation of insoluble polyamino acid chains greater than 7 units long. 

The value of the catalytic transfer hydrogenation route is demonstrated by the selective, high yield and rapid reduction of nitro aliphatic compounds to their corresponding amine derivatives using anhydrous ammonium formate (equation 26). A wide variety of nitro compounds are reduced in the presence of other functional groups including acids, esters and nitriles. Furthermore, the method is stereospecific and proceeds with retention of configuration pure racemic syn-nitro alcohols (39a) and (39b) were converted to the 5yn-amino alcohols (40a) and (40b) and the axial nitrosteroid (41a) afforded the 63-amine (41b). 

Most of the enzymatic peptide forming reactions are strictly stereo-specific for L-amino acids so that racemization that often accompanies chemical coupUng of optically active amino acids does not occur. The formation of only the L-amino acid derivative out of a D,L-mixture, in an enzymatic formation e.g. of an anilide from a D,L-Z-amino acid ester and aniUne, makes proteolytic enzymes useful reagents for the resolution of racemic mixtures. This is supplementary to the enzymatic stereospecific deacylation of D,L-iV-acylamino acid mixtures where exclusively the L-derivative will be deacylated. 

Sec. alcohols from ketones with formation of lactams from aminocarboxylic acid esters Stereospecific reduction... 

Treatment of trans,trans-2,6-ociddiene (122) with deuteriated formic acid HCO2D in the presence of deuteriosulphuric acid gave the cyclized formate ester 123. A concerted mechanism (equation 63) was proposed for this reaction73. The stereospecific ring-closure of the 1,4-cyclohexadiene derivative 124 in acetic anhydride/perchloric acid affords the octalin 125, which was isolated as the diacetate 126 (equation 64)74. 

Syntheses of naphthyridone derivatives follow the same procedures as those of quinolones, except that substituted 2-aminopyridines (Gould-Jacobs modification) or substituted nicotinic ester/nicotinoyl chloride are used instead of anilines or o-halobenzoic acid derivatives. Most of the recently introduced quinolone antibacterials possess bicyclic or chiral amino moieties at the C-7 position, which result in the formation of enantiomeric mixtures. In general, one of the enantiomers is the active isomer, therefore the stereospecific synthesis and enantiomeric purity of these amino moieties before proceeding to the final step of nucleophilic substitution at the C-7 position of quinolone is of prime importance. The enantiomeric purity of other quinolones such as ofloxacin (a racemic mixture) plays a major role in the improvement of the antibacterial efficacy and pharmacokinetics of these enan-... 

An alternate approach comprises replacing the pendant sugar by either a carbo-cyclic or a heterocyclic ring. The enantioselective synthesis starts by formation of the imide (45-3) by reaction of the aion from the chiral auxiliary (45-2), derived from S-phenylalaninol and the pentene ester (45-1). Treatment of the product with triethyl amine and the trifalate from dibutylboronic acid leads to the transient enol borate (45-4). Aldol addition of that enol to acrolein proceeds stereospecifically to the alcohol (45-5) due to the transfer of chirality from the chiral auxiliary. 

Mayer and co-workers improved the amide bond formation of TenBrink s method using unprotected amino alcohols and active esters of bromoalkyl carboxylic acids.[5] More recently, Anthony et al.,[6] and Norman and Kroin[7l reported stereospecific alkylation of acylmorpholinone (Schemes 3 and 4) using sodium hexamethyldisilazanide and lithium diisopropylamide, respectively. 

The number of photoelimination reactions that occur in a concerted manner is very small. One of the best-characterized examples does not lead to the formation of a C—C bond between the two a-carbons. Rather, it involves the stereospecific photoelimination of CO from 3,5-cydoheptadienone 15 to 1,3,5-octatriene 16 (Scheme 2.5) [18]. The reaction occurs in a stereospecific manner with the relative stereochemistry of the two a-carbons determining the configuration of the resulting 1,5-double bonds. Another interesting and more relevant example involves the photodecarboxylation of enantiomerically pure aromatic esters derived from (+ )-or (—)-2-methylbutyric acid and 2,4,6-trimethylphenol 17, which occurs with 100% retention of configuration in the product 18. [19] While this is a promising lead that... 

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